1. Field of the Invention
The present invention relates to a rotary movement converting mechanism for converting a rotary movement of a rotary body into a linear movement of a movable body, and a measuring instrument for measuring a displacement of a spindle in a linear direction.
2. Description of Related Art
Conventionally, a micrometer has been used as a measuring instrument for measuring a dimension of a workpiece by measuring a linear displacement of a spindle with an end of the spindle being abutted to the workpiece (see U.S. Pat. No. 5,495,677, FIG. 3).
The micrometer shown in the U.S. Pat. No. 5,495,677 has an inner sleeve fixed on a body frame, an outer sleeve provided on the outer circumference of the inner sleeve, a spindle, and a top member attached to the spindle. The spindle works as a support body having an end fixed to the body frame and a slit is formed on the circumference thereof along the moving direction of the spindle. The top member is attached to an end of the spindle, the top member having an engaging member projecting in a radial direction of the outer sleeve. An end of the engaging member is engaged with a spiral grove formed on the outer sleeve. The outer sleeve works as a rotary body, and the spindle (movable body) linearly moves in accordance with the rotation thereof with the rotation thereof being restricted.
According to the micrometer of the U.S. Pat. No. 5,495,677, in order to engage the end of the engaging member to the spiral groove formed on the outer sleeve in assembling the components, a clearance is necessarily formed between the spiral groove and the engaging member. Accordingly, when a load is applied on the spindle while the spindle is abutted to a workpiece, the top member is minutely displaced by the distance of the clearance. The spindle is also displaced in accordance with the minute displacement of the top member, which results in a measurement error. Since the clearance facilitates the movement of the engaging member, if the clearance is eliminated, the spindle is not smoothly moved on account of strong abrasion between the spiral groove and the engaging member.